Fluid exchange in single capillaries will be measured in the isolated mesentery preparation when the input and output pressures are equilibrated at different predetermined levels. The measurements will be carried out by manually tracking the trajectories of red blood cells in mechanically occluded capillaries and by monitoring the changes in concentration of fluorescine tagged macromolecules. The following methodology will be 1) Red blood cell tracking from video recording. The relative position of red cells will be measured by juxtapositioning video cursors to cells being tracked and electrically displaying their separation on a chart recording. The procedure will be carried out on a stop frame basis every 10th frame (1/6 second); 2) Protein density measurements by photometric techniques in a single microvessel; 3) Preliminary design work will be carried out to implement a microplethysmograph which can be used in conjunction with the microscopic intravital studies, in which fluid exchange is measured directly in single microvessels. BIBLIOGRAPHIC REFERENCES: Intaglietta, M., and J.F. Gross: "Microscopic and macroscopic relationships between blood flow and fluid exchange," Proc. Summer Computer Simulation Conf., Simulation Councils Ed., La Jolla, Cal, 869-877, 1975. Mahler, F., M. Intaglietta, T.E. Hugli, and A.R. Johnson: Influences of C3a anaphylatoxin compared to other vasoactive agents on the microcirculation of the rabbit omentum," Microvasc. Res. 9: 345-356, 1975.